.App {
  text-align: center;
  padding: 100px;
  position: relative;
}

.cus-panel {
  height: 300px;
  padding: 20px 10px;
  border: 1px solid red;
}

.operation-panel {
  padding: 30px 0;
}
.operation-panel .ct {
  height: 240px;
  text-align: center;
  display: flex;
  flex-direction: column;
  justify-content: space-between;
  border: 1px solid red;;
}

.cus-panel .ct {
  height: 100%;
  overflow-y: scroll;
  text-align: left;
  border: 1px solid black;
}

.cus-panel .ct li {
  list-style: none;
  height: 25px;
  line-height: 25px;
  cursor: pointer;
}

.cus-panel .ct li:hover, .cus-panel .ct li.active {
  background-color: #eee;
}

.option-btn {
  height: 30px;
  width: 50px;
}

.mail-panel {
  padding-top: 20px;
}

.mail-panel .ct {
  overflow-y: scroll;
  height: 300px;
  margin-top: 20px;
  padding: 10px;
  text-align: left;
  border: 1px solid #aaa;
}

.clearfix ::before, .clearfix::after {
  display: table;
  content: "";
}
.clearfix::after {
  clear: both !important;
}